1. Field of the Invention
The present invention relates to a developing apparatus for developing an electrostatic image that has been formed through an electrophotographic printing method or an electrostatic recording method on an image bearing member.
2. Description of the Related Art
Up to now, in an image forming apparatus such as a copying machine which adopts an electrophotographic printing method, an electrostatic image formed on an image bearing member such as a photosensitive drum is visualized by adhering a developer onto the electrostatic image. For the developer, a magnetic one-component developer containing magnetic toner, a non-magnetic one-component developer containing non-magnetic toner, two-component developer containing non-magnetic toner and a magnetic carrier, and the like may be appropriately used.
As an example of conventional developing apparatuses using such the developers, a developing apparatus using the two-component developer containing non-magnetic toner and a magnetic carrier is illustrated in FIG. 7.
As shown in FIG. 7, a developing apparatus 1A using the two-component developer often has a structure of a single-sleeve developing apparatus. In other words, the developing apparatus 1A includes a developer carrying member for carrying and feeding the developer to develop an electrostatic image formed on a photosensitive drum. A developer carrying member 8 includes a developing sleeve 8a and a magnet roll 8b which is arranged so as to be fixed within the developing sleeve 8a. Further, the developing apparatus 1A stores the two-component developer in a developer container 2 and includes feeding screws 5 and 6 within the developer container 2 serving as feeding means for feeding the developer to the developing sleeve 8a while agitating the developer.
However, in the developing apparatus 1A, there is a possibility to be generated a blank area image which is one of image defects caused by an edge enhancement.
Hereinafter, the blank area image generating mechanism will be described with reference to FIG. 8. It should be noted that this is an example of a developing apparatus which adopts a reversal development method.
In general, in an electrostatic image formed on a photosensitive drum 10, when an image containing a halftone image is formed, the blank area image is generated in the vicinity of a border between a highlight image α which is formed at a downstream of a movement direction of a surface of the photosensitive drum 10, and a solid image β which is formed at an upstream thereof in a developing region. That is, the blank area image is generated between a trailing end of the highlight image α and a leading end of the solid image β.
FIG. 8 shows configurations of an equipotential surface and electrical lines H of force when a highlight portion α is provided on the photosensitive drum 10, a solid portion β is provided in a backward direction of the portion α, and a border portion γ between the highlight portion α and the solid portion β is opposed to the developing sleeve 8a. As apparent from FIG. 8, the electrical lines H of force is moved toward the solid portion β to a large extent in the vicinity of the border portion γ.
Thus, by using the developing method with a conventional structure in which the developing sleeve 8a is rotated in a forward direction with respect to the photosensitive drum 10, it is not possible to supply the trailing end of the highlight portion α with toner contained in the developer to be supplied. As a result, the development is performed along the electrical lines H of force toward the solid portion β. Therefore, it is presumed that a blank portion occurs at the trailing end of the highlight portion α.
Thus, in order to prevent the blank area image from occurring, a twin-sleeve developing method using the two-component developer has been proposed as shown in FIG. 9 (for example, see Japanese Patent Application Laid-Open No. H10-171252 and Japanese Patent Application Laid-Open No. 2003-323052).
A developing apparatus 1B adopting the twin-sleeve developing method includes two developer carrying members 8 and 9 at an upstream side and a downstream side of a rotating direction of the photosensitive drum 10, that is, the developing sleeves 8a and 9a including magnet rolls 8b and 9b within the developer container 2. Through a first developing process which is performed by using the developing sleeve 8a provided at the upstream side and a second developing process which is performed by using the developing sleeve 9a provided at the downstream side, the identical electrostatic latent images formed on the photosensitive drum 10 are visualized.
The twin-sleeve developing method is a developing method in which a potential difference between the highlight portion α and the solid portion β is reduced in the first developing process described above and the development is reliably performed at the trailing end of the highlight portion a in the second developing process, thereby preventing the blank area image from occurring.
However, there arises the following problem in the structure of the developing apparatus 1B adopting the twin-sleeve developing method.
Referring to FIG. 9, a description as to the problem in the twin-sleeve developing method will be given.
In the developing apparatus 1B, a delivery of a developer T is performed by a magnetic pole N3 of the magnetic roll 8b fixed within the developing sleeve 8a and by a magnetic pole S3 of the magnetic roll 9b fixed within the developing sleeve 9a. The developer once delivered onto the developing sleeve 9a is fed by the rotation thereof. However, it depends on each condition of the magnets within two sleeves that a part of the developer T is delivered again onto the developing sleeve 8a, so the developer T is likely to be retained in a portion between the developing sleeve 8a and the developing sleeve 9b. In a portion between the developing sleeve 8a and the developing sleeve 9b, the developer T is subsequently fed by the developing sleeve 8a. When the retained developer is excessively increased, there is a possibility that the retained developer is brought into contact with the photosensitive drum and disturbs the toner image formed on the photosensitive drum.
There is another possibility to cause a phenomenon that the developer T receives a large pressure due to the above-mentioned retention, fine particles such as titanium oxide externally added to the toner is embedded in the toner. Further, due to a friction between the toner and the magnetic carrier, a particle configuration of the toner itself may be rounded off. When the developing apparatus is used for a long period of time, a phenomenon that the toner is adhered to a surface of the magnetic carrier and is hard to be scraped off, so-called a spent phenomenon, may be caused. Thus, when a deterioration of the developer is caused, a triboelectrification amount (hereinafter, referred to as “triboelectricity”) is changed according to a period of time in which the developing apparatus is used. Due to the change, an image density may be changed in association with a change of a developing performance of the toner, or a mechanical adhesion of the toner with respect to the magnetic carrier or the photosensitive drum is increased, whereby development or transferring corresponding to an electric field is less likely to be performed. As a result, a partial defect (i.e., unevenness) of the toner is caused, thereby forming an image making an extremely bad impression after the usage of the developing apparatus for a long period of time.
After a study by the inventors of the present invention, it becomes apparent that the above-mentioned problem is closely related to positions of the magnetic pole N3 of the magnet roll 8b and the magnetic pole S3 of the magneto roll 9b, and to a magnetic force generated by these magnetic poles.
For example, when an idling of the developing apparatus is performed for an hour by setting a magnetic force of the magnetic pole S3 as 1.5 times of the magnetic pole of N3, it becomes apparent that the developer is not retained in a portion between the developing sleeves. In addition, when a position of the magnetic pole S3 is moved by an angle of 5° to a downstream side of a rotating direction of the developing sleeve 9 without changing the magnetic force, the same result has been obtained.
The developing apparatus 1B has a structure in which an attraction of the developer in the developing sleeve 9a is enhanced in order to prevent the retention of the developer in a portion between the developing sleeves 8a and 9a. However, in the structure of the positions of the magnetic pole N3 and the magnetic pole S3, and the configuration of the magnetic force, there arises the following problem. That is, as shown in FIG. 10, the developer is delivered from the magnetic pole S2 to the magnetic pole N4 not through the magnetic pole N3 and the magnetic pole S3, thereby the developer is retained like a bridge. This is a problem of a so-called “bridge phenomenon of a developer”.
When the bridge phenomenon of the developer is generated, the photosensitive drum 10 and the developer T are rubbed over a wide range. As a result, the toner image formed on the photosensitive drum 10 which is formed into the toner image by the developing sleeve 8a is scraped off, thereby causing so-called “scavenging phenomenon” to remarkably deteriorate an image level.
As described above, even when the magnetic field for promoting the delivery of the developer is formed to prevent the retention of the developer in a developer delivering part of two sleeves, in a case where the “bridge phenomenon of the developer” is caused in another place, a disturb of the toner image is finally caused.
After the study by the inventors of the present invention, the above-mentioned problem is caused by the following reason.
That is, a magnetic coupling between the magnetic pole N3 of the magnetic roll 8b and the magnetic pole S3 of the magnetic roll 9b is promoted, and to the contrary, a magnetic coupling between the magnetic pole S2 and the magnetic pole N3 is weakened. As a result, magnetic force lines are newly generated in each of the magnetic pole S2 and the magnetic pole N4.
In a case where the distance between the developing sleeves 8a and 9a is increased to thereby obtain a configuration in which magnetic force lines are not easily generated, the “bridge phenomenon of the developer” has not been generated when an idling of the developing apparatus is performed for an hour.